Systematics and Evolution, Part A The Mycota

tures (Moss 1985 , 1986 ). A third group, the
aplanochytrids, superficially resemble thrausto-
chytrids but are able to glide slowly along ecto-
plasmic threads. The slime tracks and feeding
rhizoids are connected to the thallus by means
of a complex plug structure (Fig.3.3b), vari-
ously known as thesagenogenetosome(Moss

1985 )orbothrosome (Porter 1990 ). Some

thraustochytrid thalli proliferate internally, as

shown inT. rossi(Fig.3.3d–f) (Karling 1981 ),

whilst in genera such asSchizochytrium, the

thalli divide by successive bipartitions

(Fig.3.2). Epibiotic resting spores are produced

by some species, although it has not been

Fig. 3.3 (a–k) Morphology of Labyrinthulomycota.
(a) Line drawing illustrating ultrastructure of uninucle-
ateLabyrinthulacells enclosed by membranes of ecto-
plasmic network. Adapted from Porter ( 1990 ). (b)
Ultrastructural detail of bothrosome (sagenogeneto-
some) complex separating labyrinthulid thallus from
ectoplasmic strand in Aplanochytrium (formerly
Labyrinthuloides)minutum, showing continuity in cis-
ternae across complex. Adapted from Perkins ( 1976 ).
(c) Diagram illustrating ultrastructure of immature
uninucleate thallus ofThraustochytriumshowing finely

branched basal ectoplasmic network and cell coat of

consolidated scales. Adapted from Porter ( 1990 ). (d–g)

Thraustochytrium rossion pollen showing internal pro-

liferation of basal thalli (d–f), zoospore discharge (f),

and biflagellate heterokont zoospores (g). (h,i)Ulkenia

amoeboidaon pollen showing mature thallus (h), a

discharging amoeboid protoplast (i) that divides to

form individual amoebospores (j) that upon settling

release heterokont zoospores (k). Adapted from Kar-

ling ( 1981 ) based on observations of Bahnweg and

Sparrow ( 1974 )

Systematics of the Straminipila: Labyrinthulomycota, Hyphochytriomycota, and Oomycota 45